1. Field of the Invention
The invention concerns a method for automatic hardness testing of materials according to Vickers, in which varying brightness in an image of an indentation produced in the material is photoelectrically scanned and evaluated in order to determine the length of the diagonal of the indentation. The invention further concerns apparatus for carrying out the method.
With the method and apparatus of the invention, the hardness of a material is determined by measuring the dimensions of an indentation produced in the material by an indentor in order to ascertain the depth of penetration of the indentor.
2. The Prior Art
The Vickers method of hardness testing involves pressing the apex of a square-pyramid-shaped diamond having a predetermined apical angle into the material to be tested, under a load. The Vickers hardness number is then calculated according to the formula: EQU V=P/0.5393 (1).sup.2
where P is the imposed load in kilograms and 1 is the diagonal of the indentation in millimeters.
To measure such indentions, it is known from U.S. Pat. No. 3,822,946 to scan the indentation produced in the material by the indentor with an optical beam and to use a photoelement to produce electrical signals from the multiple reflections of this beam, the amplitude of the electrical signals being proportional to the dimension of the indentation following logic processing.
Proceeding from densitometric processes, it is known from German patent DE-PS No. 721 918 to pass the slit of a selenium cell photometer over the indentation, while applying dark field illumination, to measure such indentations in metallic materials and to use the change in surface brightness as a mark of the beginning and end of the gauge length.
Both of these arrangements require illuminating equipment with--as described in the case of German patent DE-PS No. 721 918--special provisions. The technical outlay for auxiliary apparatus is thus very substantial.
German published application DE-OS No. 27 37 554 describes a device for hardness testing having means for measuring the diagonal length of the indentation, wherein an image converter comprising a plurality of photoelectric converter elements is arranged in an image plane upon which a bright and dark image of the indentation is effected by means of an optical device. A unit is provided for determining the diagonal length of the indentation in dependence upon the output signal of the image converter.
A precondition for the attainment of accurate measuring results is, however, homogeneity of the photoelectric converter elements of the image converter, which is nearly unattainable from a manufacturing standpoint. This leads either to potentially false measuring results or to costly selection of converter elements.